1. Field of the Invention
This invention relates to a system and method for key delivery using quantum cryptography. The system allows for delivery of single photons from a transmitter to a receiver, with information contained in the polarization states of the photons. The system eliminates expensive, unstable, complicated components, called Pockels cells, used in prior designs and replaces them with less expensive passive components and additional optical sources. Benefits arising from use of the system of this invention include system speed, stability and simplicity. In addition, the receiver design permits single photon operation in daylight conditions.
2. Description of Prior Art
Generally, quantum cryptography is a system which enables two parties to communicate in secret with the security of the communication being guaranteed by the laws of quantum physics. More particularly, a quantum cryptography system is a key distribution system that attempts to link the security of the system to the correctness of the uncertainty principle of quantum mechanics. The essence of the uncertainty principle is twofold. First, any measurements made on a physical system that extracts some information about that system will necessarily disturb that system albeit perhaps only in a minimal way. Second, any measurement made on a physical system that extracts some information about a certain quantity, call it x, necessarily precludes obtaining information about a conjugate quantity of the same system, call it p. Quantum cryptography systems are designed such that a sender prepares a physical system in a known quantum state of x or p and transmits it to a legitimate receiver. At the receiver, the value of either x or p for the physical system is measured; however, due to the uncertainty principle, the measurement of the values of both x and p is precluded. A large number of such exchanges between the transmitter and the receiver are made after which a comparison is made between the information sent and the information received. Information received by the receiver in a measurement of the conjugate quantity to that sent by the transmitter is discarded. In addition, events of unsuccessful transmission, due to the failure of arrival of the quantum state or the arrival of multiple quantum states, are discarded. In the absence of an eavesdropper, as well as the use of ideal equipment, the values of the quantities for each of the retained pieces of information would be common to both the transmitter and the receiver and can then be used as a key. Due to the uncertainty principle, if an eavesdropper extracts some information about the system by making a measurement, the system will be perturbed and the existence of the eavesdropper known.
Applications of such a system have been proposed for key delivery to satellites, aircraft, ships, and submarines, where physical delivery of a key is impossible. As computing power increases, standard mathematical encryption algorithms become susceptible to attack. However, the quantum uncertainty built into the system of this invention is completely unbreakable. As a result, the system has applicability in military, government and financial communication systems.
More particularly, a quantum cryptography system operates by transmitting key information in the form of the polarization state of a single photon. For example, a "0" bit could be transmitted in a horizontally polarized direction while a "1" bit could be transmitted in a vertically polarized direction. If one measures the photon polarization at the receiver in the same basis as the transmitter, the correct bit value is found. However, if one measures the photon polarization in a basis that is rotated (for example at 45.degree. to the correct basis), then the precise bit value cannot be determined. Photons in one of two non-orthogonal basis systems are then transmitted with the polarization direction in the system defining the bit value. A measurement on the polarization state of the photon will destroy information on the input polarization state as a result of which any attempt to eavesdrop on the system will perturb the system in a detectable manner.
A conventional system for quantum key delivery comprising transmitter 10 and receiver 20 is shown in FIG. 1. Here, a single optical source, such as a pulsed laser diode 11, generates a weak pulse, with a mean photon number per pulse of one or less after passing through linear polarizer 13. A random digital signal (on or off) is sent to a half-wave Pockels cell 12 which either transmits the original polarization or rotates the polarization direction by 90.degree.. This cell is called the bit-value selection cell 14 because it determines whether a 0 or a 1 is sent. A second random digital signal is sent to a quarter-wave Pockels cell which either transmits the input polarization or converts the polarization to a circular polarization state. The direction of circular polarization (left or right) depends on the bit value. This second cell is referred to as a transmitter basis selection cell 15. At receiver 20, a third, quarter-wave Pockels cell 12 is used. When a signal is applied to this cell, it converts circular polarization to linear polarization, and vice versa. With no signal applied, the polarization state is unchanged in propagation through the cell. This cell serves to determine the polarization basis that can be separated by the following polarizing beamsplitter 22 and, thus, this cell is referred to as a receiver basis selection cell 21. With the correct basis selected, the polarizing beamsplitter 22 directs the photon to the photomultiplier tube 23 corresponding to the correct bit value. If the incorrect measurement basis is used, either photomultiplier tube 23 will detect the photon with equal probability, yielding no information on the photon state.
This conventional system suffers from several drawbacks. First, the maximum data rate is limited by the switching speed of the Pockels cells 12, which is typically 1 Mb/s. Secondly, the Pockels cells phase retardation (and, thus, polarization control) is highly temperature dependent, drifting significantly with environmental changes. Thirdly, the Pockels cells and their driver electronics are quite expensive, with prices for a single unit near $3,500. Finally, because the system uses single photons, all ambient light must be blocked from the photomultiplier tubes 23 used as detectors, requiring near-total darkness for operation.
A similar system to that shown in FIG. 1 is taught by U.S. Pat. No. 5,243,649 in which a pulsed laser produces a single photon by attenuating a high-intensity pulse and, using Pockels cells, one observer rotates the polarization of the photons through a selected angle of rotation while a second observer measures the polarization and obtains total correlation (same polarization). Thus, any interception of the photon will destroy the correlation. This system design incorporates four (4) Pockels cells, polarization preserving optical fiber, and a feedback loop for phase drift compensation.
Other key distribution systems using quantum cryptography are taught, for example, by U.S. Pat. No. 5,675,648 in which a common transmission medium is used for the quantum channel and the public channel and a calibration signal is transmitted over the public channel on the common transmission medium to calibrate the system for subsequent transmission of a key on the quantum channel. In accordance with the teachings thereof, the transmission medium may be an optical fiber and the transmitter may switch between a single photon output and a multiple photon output to provide the quantum channel and the public channel, respectfully. U.S. Pat. No. 5,307,410 teaches an interferometric quantum cryptographic key distribution system comprising a quantum channel for conveying dim and bright reference light pulses, a timing channel, a source of coherent light pulses, beam supporters, a random number generator, a phase modulator, and a memory for recording the phase of transmitted dim light pulses; and U.S. Pat. No. 5,339,182 teaches a method and apparatus for quantum communication employing non-classical correlations of quadrature-phase amplitudes where, in order to achieve non-classical transmission effects, a nondegenerate optical parametric amplifier is employed to generate a pair of signal beams having a highly correlated quantum noise and information is then encoded within each of the signal beams at a signal level far below the level of the quantum noise. As a result, information encoded on the beam is substantially obscured by the quantum noise, thereby effectively preventing unwanted eavesdropping on the transmitted information. After transmission, the pair of separate signals are detected and combined by a receiver. The highly correlated quantum noise is eliminated from the signals, leaving substantially only the encoded information. The resulting information signal has a minimum noise level below that which can be achieved using non-classical techniques.